In the manufacture of optical fibers, a glass fiber is produced by spinning molten glass, and a resin coating is provided over the glass fiber for protection and reinforcement. This step is referred to as “fiber drawing”. As the resin coating, a structure is known in which a flexible primary coating layer is formed on the surface of the optical fiber and a rigid secondary coating layer is applied over the primary coating layer. A structure is also known in which the resin-coated optical fibers are placed side by side on a plane and bundled with a bundling material to produce a ribbon-shaped coating layer. A resin composition for forming the primary coating layer is called a primary material, a resin composition for forming the secondary coating layer is called a secondary material, and a resin composition for forming the ribbon-shaped coating layer is called a ribbon matrix material.
The outer diameter of the resin-coated optical fiber is usually about 250 μm. The outer diameter is increased to about 500 μm by applying an additional resin layer to the resin-coated optical fiber in order to improve manual workability. Such a resin coating layer is usually called an “upjacket layer”. Since the upjacket layer does not require optical properties, the upjacket layer need not have transparency. The upjacket layer may be colored for identification by naked eye observation. It is important that the upjacket layer be easily removed from the resin-coated optical fiber without causing damage to the underlying primary or secondary coating layer when connecting the resin-coated optical fibers.
A curable resin used as the optical fiber coating material, including the material for the upjacket layer, is required to have superior coatability which allows high speed fiber drawing; sufficient strength and flexibility; excellent heat resistance; excellent weatherability; superior resistance to acid, alkali, and the like; excellent oil resistance; small degrees of water absorption and hygroscopicity; low hydrogen gas generation; excellent liquid storage stability; and the like.
However, since a related-art upjacket material firmly adheres to the overlying ribbon matrix material layer or underlying primary or secondary coating layer, the upjacket layer may be damaged when removing the ribbon matrix material layer to expose the resin-coated optical fiber, or the primary or secondary coating layer may be damaged when removing the upjacket layer from the resin-coated optical fiber. This hinders optical fiber connection workability.
As curable liquid resin optical fiber upjacket compositions provided with improved removability, a composition containing three types of polysiloxane compounds (patent document 1), and a composition containing organic or inorganic material particles (patent documents 2 and 3) have been disclosed.    [Patent document 1] Japanese Patent Application Laid-open No. 10-287717    [Patent document 2] Japanese Patent Application Laid-open No. 9-324136    [Patent document 3] Japanese Patent Application Laid-open No. 2000-273127
However, an upjacket layer formed by using the above-mentioned composition exhibits insufficient removability.